1. Field of the Invention
The technology disclosed herein relates to a transfer device and an image forming apparatus that transfer a toner image formed on an intermediate transfer member onto a recording paper sheet by applying an alternating voltage superimposed on a direct current voltage.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a development method has been employed in which a surface of an electrostatic latent image bearing member (for example, a photoreceptor) is charged and an image is exposed to the charged region to from an electrostatic latent image, and the electrostatic latent image is developed so as to be made visible (developing).
As such a development method, a development method has been commonly used in which, using one-component developer containing a toner or two-component developer containing a carrier and a toner, by frictionally charging the toner so that the toner is attracted with an electrostatic force of an electrostatic latent image on the surface of the electrostatic latent image bearing member, the electrostatic latent image is developed to thereby form a toner image.
A toner image formed on the electrostatic latent image bearing member is transferred again onto an intermediate transfer member, which is a drum-shape or a belt-shape, with an electrostatic force. The toner image transferred onto the intermediate transfer member is to be further transferred onto the recording paper sheet with the electrostatic force.
Finally, the recording paper sheet is conveyed to a fixing device to fix the transferred toner image on a surface of the paper by applying heat and pressure, and thus paper on which an image is printed is obtained.
In such an image forming apparatus, it is desired to form an image of smooth image quality with little roughness.
Furthermore, it is desired to obtain the similar image quality not only under the normal temperature but also even with conditions of high temperature and high humidity, or conditions of low temperature and low humidity, and it is desired to obtain the similar image quality for not only the plain paper but also for various types of recording paper sheet such as heavy paper or unlevel embossed paper.
However, for example, when a two-component developer is used, a charge amount held by a toner is easily changed depending on the surrounding environment or the using situation and thus is unstable as transferring performance using the electrostatic force, therefore, it is difficult to transfer the toner image on the intermediate transfer member onto the recording paper sheet at a rate of 100%.
Therefore, various approaches have been taken conventionally for the improvement of such a transfer property. For example, a method that one in which direct current bias and alternating current bias are superimposed is applied as a secondary transfer bias to a primary transfer image which has been transferred onto the intermediate transfer member, to be transferred onto the recording paper sheet, has been proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 9-146381 (1997)).
In the image forming apparatus described in JP-A 9-146381, although it is described as excellent in a cleaning property of the intermediate transfer member and in transfer efficiency from the intermediate transfer member to the recording paper sheet, and especially there will be no occurrence of local transfer failure, it is not possible to form a sufficiently high quality image since as a bias waveform to be used, a constant alternating current component is merely superimposed on a direct current component.